Specific binding of 1-3H-dihydroalprenolol to beta-adrenergic receptor binding sites in rabbit whole lung membrane fragments has been demonstrated. The binding characteristics of several physiologically non-selective and selective beta-adrenergic receptor agonists and antagonists were altered by incubation temperature (37 degrees C or 25 degrees C); these changes were not consistent with receptor subtype interconversion. Such changes in binding parameters can lead to inaccurate determination of the presence and distribution of receptor subtypes in a tissue; conversely, the physiological selectivity of some agents may not be recognized based on binding characteristics alone. The binding of 3H-clonidine to rabbit lung membrane fragments was complex. In preliminary experiments, the relative potencies of several competing agents did not correlate with physiological data. Radioligand-receptor binding to both alpha- and beta-adrenergic receptors was also examined in the isolated, perfused rabbit lung preparation. No evidence in support of specific receptor binding of the radioligands, 1-3H-dihydroalprenolol, 1-3H-epinephrine and 3H-clonidine, was observed in spite of measurable physiological responses (e.g., increased perfusion pressure) by several competing agents. The lack of demonstrable specific receptor binding was apparently due to extensive non-specific sequestration of each of the three radioligands tested. In view of these difficulties, radioligand-receptor binding in intact tissue will be examined in isolated pulmonary vessels and membrane fragment preparations in which non-specific uptake and binding of radioligands are expected to be less extensive than in the intact lung and specific receptor binding more easily demonstrable.